Mr Johnson has been under pressure since writing a piece for the Daily Telegraph in which he suggested women who wear face veils look like “letterboxes” and “bank robbers”.

In the article, which was arguing against the UK following Denmark and introducing a ban on face coverings, Mr Johnson also described burkas as “oppressive and ridiculous”.

The language outraged some who accused the former Foreign Secretary of stirring up Islamophobia.

Mohammed Amin, chair of the Conservative Muslim Forum, described the comments as “very unfortunate” and claimed they could boost the far-right.

The row followed the Prime Minister to Edinburgh, where she was discussing post-Brexit investments.

Asked if Mr Johnson should apologise, or if he should lose the party whip, Mrs May told reporters: "I am very clear, and the Government is very clear about our position on the burka, which is that women should be able to choose how to dress.

"It's up to women how they should dress. It's not up to other people to tell a woman how to dress.

"But obviously these issues are ones that are openly discussed and it's right that we have a discussion about issues like this."